Preparation of flaked phosphorous acid



United States Patent Int. C1. 3013' 2/00 US. Cl. 23-293- 6 Clalms ABSTRACT OF THE DISCLOSURE A process for producing a non-caking flake phosphorus acid product which comprises depositing a layer of molten phosphorous acid on a cool surface, not in excess of about 30 centigrade, maintaining said layer on the surface for a time suflicient to substantially solidify the phosphorous acid in a stable, crystalline form and, thereafter, flaking said phosphorous acid layer.

This application is a continuation-in-part of my copending application Ser. No. 446,390, filed Apr. 1, 1965.

This invention relates to an improved process for preparing phosphorous acid and more particularly relates to an improved process for treating a molten phosphorous acid to obtain a solid, flaked product.

For the most part, at the present time, phosphorous acid is prepared commercially by hydrolyzing phosphorous trichloride (PCl The resulting reaction mixture, generally as a melt of substantially pure phosphorous acid or as a concentrated aqueous phosphorous acid solution, is normally subjected to crystallization to obtain a solid phosphorous acid product. While such techniques are generally satisfactory, insorfar as the type of product obtained is concerned, it is well recognized that the crystallization processes normally require closer control of process variables as well as closer supervision during operation, than do other recovery processes. Additionally, the initial cost and maintenance of crystallizing apparatus is also, normally, more than that for other types of collecting apparatus such as centrifuges, flakers, and the like. For this reason, it is desirable if means other than crystallization can be utilized in obtaining a solid phosphorous acid product.

In the past, however, some ditficulties have been encountered with flaked phosphorous acid, principally with the flowability of the flaked product after storage. Normally, such flaking operations are carried out so as to yield a relatively small, thin phosphorous acid flake. In many instances, it has been found that upon storage, as for example in a drum or a bag, caking was evident after one or two days, and after one to two weeks, these flakes became so solidly fused together and caked that it was often impossible to remove them from the container without destroying the container. As a result of this difliculty, flaking techniques for obtaining a solid phosphorous acid product have not been widely adopted even though such processes are generally less expensive to operate and install than processes involving crystallization.

It is, therefore, an object of the present invention to provide a novel method of obtaining a flaked phosphorous acid product from molten phosphorous acid.

A further object of the present invention is to pr vide a novel method for obtaining phosphorous acid flakes from molten phosphorous acid, which flakes are characterized by their ability to remain free flowing even during long periods of storage.

Another object of the present invention is to provide 3,510,268 Patented May 5, 1970 a novel flaked phosphorous acid product, which product remains free flowing even during long storage periods.

These and other objects of the present invention will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above object, the present invention includes an improvement in the process for preparing phosphorous acid wherein the phosphorous acid is obtained as a molten product, which improvement comprises depositing a layer of molten phosphorous acid on a cool surface, the surface being at a temperature which is not in excess of about 30 degrees centigrade, maintaining said layer on the surface for a period of time sufficient to effect substantially complete solidification of the layer in a stable crystalline form wherein the layer is at a substantially uniform temperature throughout its thickness, and, thereafter, removing the layer from the surface and forming it into a flaked phosphorous acid product.

More specifically, in the practice of the method of the present invention, a layer of molten phosphorous acid is deposited on a cool surface, which surface is at a temperature which is not in excess of about 30 degrees centigrade. Preferably, the cool surface on which the layer of molten phosphorous acid is deposited is substantially smoothso that the thickness of the deposited layer is substantially uniform. Desirably, this surface is of a material which is not reactive with the molten phosphorous acid so that contamination of the solid phosphorous acid product and destruction of the surface are avoided. Exemplary of various materials which may be used are metals, such as stainless steel and nickel, glass, and the like.

The thickness of the layer of molten phosphorous acid deposited on the cool surface should be sufficient to obtain a flaked product rather than disintegrating into a dust or powder. Thicknesses of at least 0.01 inch are typical. It is important that at least this thickness of the layer of molten phosphorous acid be maintained, since at lesser thicknesses, it has not been found to be possible to obtain the desired flaked product. Desirably, the layer of molten phosphorous acid deposited has a thickness within the range of about 0. 02 to about 0.06 inch and preferably within the range of about 0.03 to about 0.05 inch. Although in some instances, even greater layer thicknesses, such as thicknesses of 0.1 inch or more, maybe utilized, with such greater thicknesses, difliculties may also be encountered in obtaining a flaked product. Accordingly, since it is important that the layer of molten phosphorous acid, after solidification, be flakable, layer thicknesses appreciably in excess of about 0.1, inch will normally not be used.

As has been noted hereinabove, the surface on which the molten phosphorous acid is deposited is at a temperature which is not substantially in excess of about 30 degrees centrigrade. Desirably, the temperature of this surface is below about 25 degrees centigrade and in many in stances it is preferable that the surface is at a temperature which is within the range of about 0 to about 20 degrees centigrade. Although surface temperatures below 0 degrees centigrade may be used with no disadvantageous effects on the product obtained, the use of such low temperatures generally do not result in a flaked product having flowability properties which are greatly improved over those of products produced using temperatures within the range of 0 to 25 degrees centigrade. This is not to say, however, that such lower temperatures may not be used but merely that, from an economic standpoint, the process of the present invention is not normally operated at temperatures below about 0 degrees centigrade.

The molten phosphorus acid layer is maintained on the cool surface for a period of time sufficient to effect substantially complete solidification of the layer in a stable crystalline form wherein the temperature of the layer is substantially uniform throughout the layer thickness. It is believed that where the layer of phosphorus acid does not have a substantially uniform temperature throughout, it is in a non-stable crystal form, which nonstable form adversely affects the storage properties of the flakes and causes caking. By having the solidified phosphorus acid layer at a substantially uniform temperature throughout, it is in a stable crystal form and the flakes produced therefrom remain flowable even after extended periods of storage. In referring to a substantially uniform temperature, it is intended to mean that the temperature differential in the layer is not more than about 1520 centigrade and preferably less than about 10 centigrade. Although the time required to affect this will vary, depending upon the thickness of the layer and the temperature of the surface on which it is deposited, it has generally been found to be desirable that the layer of phosphorus acid is maintained on the surface for a period of at least about 20 seconds. It has been found that where the surface is at a temperature within the range of about to 25 degrees centigrade and the layer thickness is within the range of about 0.02 to about 0.05 inch, retention times within the range of about 30 to about 100 seconds are sufficient to effect sunbstantially complete solidification of the molten phosphorous acid in a layer wherein the temperature is substantially uniform. Normally, retention times of less than about 20 seconds will not be used since it has been found that flaked products produced using such shorter time generally do not have the desired flowability. Additionally, retention times greater than about 100 seconds are not normally used, since the effect on the product, although not deleterious, is negiligibly beneficial. This is particularly true where it is desired to operate the process continuously and the additional retention time may measurably reduce the production rate of the flaked phosphorous acid.

In this latter regard, i.e., the continuous operation of the present process, it has been found to be desirable that the layer of molten phosphorous acid is deposited on a moving surface, such as an endless belt or a rotating drum. In this manner, the molten phosphorous acid may be continuously deposited on the surface in a layer having the desired thickness and may then be continuously removed from the surface at the end of the desired retention time. Various means of depositing the layer of molten phosphorous acid on the moving surface may be used, as are well known to those in the art. For example, where an endless belt is used, the molten phosphorous acid, from a reservoir or storage tank may be deposited on the belt by a suitable conduit or distributing pipe. Depending upon the Width of the belt, suitable baflles or other leveling means may be utilized to obtain the desired thickness of the molten layer. Where a rotating drum is used, the layer of phosphorous acid may be deposited by rotating the drum surface through a body of the molten phosphorous acid. Here again, suitable baffle or distributing means may be used to insure that the layer is of the desired thickness, although, normally this will be accomplished substantially automatically by the speed at which the drum is rotated and the depth to which it is submerged.

It is to be appreciated that in some instances the layer of phosphorous acid may be cooled by being deposited on a plurality of cool surfaces, as for example, by the use of a series of cooled milling rolls. In this instance, the acid layer is transferred from one cooled roll to another. In so doing, since the surface of the layer in contact with the rool is reversed by the transfer, more rapid solidification and attaining of a substantially uniform temperature throughout the layer may be obtained.

Insofar as the length of the retention time of the layer of phosphorous acid on the moving surface, this will, of course, be governed by the speed at which the surface is moving. In the case of an endless belt, this will be the speed of the belt, while with a drum it will be the rotational speed of the drum. Generally, it has been found that where the moving surface on which the molten phosphorous acid is deposited is a rotating drum, rotational speeds of less than about 2 revolutions per minute, and preferably from about /2 to l revolution per minute, Will provide the retention time desired to effect substantially complete solidification of the molten phosphorous acid asa layer of substantially uniform temperature.

Once the substantial solidification of the molten phosphorous :acid on the surface has been effected, the solidified phosphorous acid is removed from the surface and formed into a flaked product. Preferably, the removal and flaking of the phosphorous acid is accomplished substantially simultaneously, as for example by means of a scraper or doctor blade. The flakes of phosphorous acid thus-produced are substantially the same thickness as the thickness as the thickness of the layer of molten phosphorous :acid initially deposited, i.e., preferably they have a thickness of at least 0.01 inch. Typically, these flakes have a width from about to about /2 inch and are up to about 3 inches in length. With regard to these flake sizes, it has been found that, insofar as the flowability of the flaked product after storage is concerned, the larger flakes, e.g., those having a Width of from about A; to /2 inch and a length of from about /2 to 3 inches, are preferred.

In the preferred operation of the method of the present invention, molten phosphorous acid at a temperature within the range of about 70 to about 100 degrees centigrade is deposited on a rotating drum by passing the drum through a pool of the molten phosphorous acid. Cooling Wateris introduced into the drum to maintain the surface of the drum at a temperature within the range of about 0 to about 15 degrees centigrade. The drum is rotated at a speed of about 1 revolution per minute so that there is deposited upon the drum a layer of molten phosphorous acid having a thickness within the range of about 0.03 to about 0.05 inch. At this rotational speed, substantially complete solidification of phosphorous acid is obtained and a solidified layer of substantially uniform temperature is removed from the drum by means of a scraping blade after about 0.8 revolution. The molten layer of phosphorous acid is, thus, maintained on the drum for a period of about 50 seconds. Upon removing the solidified phosphorus acid from the drum, by means of a scraper blade, there is obtained a flaked product having flakes of a thickness of about 0.04 inch, a width of about A; to 4 inch and a length of about /z to 1% inches. These flakes are translucent, have a glossy surface and are free-flowing. After being packed in containers, the flowability of these flakes was retained even after storage periods of more than five months.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the folloiwng specific examples are given. It is to be understood, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof. In these examples, unless otherwise indicated, temperatures are in degrees centigrade.

EXAMPLE 1 Molten phosphorous acid is maintained at a temperature of about degrees centigrade in the feed tray of a drum flaker. The flaker drum, which is about one foot in diameter and about eighteen inches in length, is cooled, by the introduction of cooling water, to a temperature of about 10 degrees centigrade. The drum is rotated through the molten phosphorous acid in the feeder tray at the rate of one revolution per minute and there was deposited on the drum a layer of molten phosphorous acid having a thickness of about 0.03 inch. This layer of phosphorous acid was maintained on the drum for a period of about 50 seconds and was then removed from the drum, by means of a doctor blade scraper, in the form of flakes of phosphorous acid. These flakes had a thickness of about 0.03 inch, a width of from about A; to A inch and a length of up to about 1 inches. After several additional runs were made. In these runs, the prcedure followed was the same as that set forth in the proceding examples. Using this procedure, the conditions under which the runs were made and the results obtained removal from the drum, these flakes were packed in fiberwere as follows:

H P0 tem- Drum tem- Layer Flake conperature Layer perature retention Flake dition after in degrees thickness 1n degrees Drum speed time in thickness 1 wk. Ex. centigrade in inches centigrade in r.p.m. seconds in inches storage.

70 0. 04 50 )4 100 0. 04 Caked. 70 0. 01 50 2 0. 01 D0. 70 0. 008 2 7 7 0. 008 D0. 91 0. 008 8 8 6 0.008 Do.

board drums and the drums were sealed. The filled drums weighed about 300 pounds. After one week of storage, the drums were opened and the flakes were found still to be free-flowing with substantially no evidence of caking in the drum. Flakes which were stored for 5 months were found to be similarly free flowing.

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the flaking drum was cooled to a tem perature of about 25 degrees centigrade and the drum was rotated at /2 revolution per minute to provide a retention time of the phosphorous acid layer on the drum of about 100 seconds. The layer of molten phosphorous acid deposited on the drum had a thickness of about 0.04 inch. The flaked phosphorous acid removed from the drum had a thickness of about 0.04 inch, a width of from A to /2 inch and a length of up to 3 inches. After five weeks of storage, these flakes were still free-flowing and easily removed from the container.

EXAMPLE 3 The procedure of Example 1 was repeated with the exception that the flaking drum was cooled to a temperature of about 0 degrees centigrade and the drum was rotated at about 1 /2 revolutions per minute to provide a retention time of the molten phosphorous acid on the drum of about 33 seconds. The layer of molten phosphorous acid deposited on the drum had a thickness of about 0.02 inch. The flakes removed from the drum had a thickness of about 0.02 inch, a width of about to inch and a length of up to about /2 inch. After five weeks of storage, these flakes were still free-flowing and easily removed from the storage container.

The procedure of Example 1 was repeated with the exception that the flaking drum was cooled to a temperature of 22 degrees centigrade, the other conditions remaining the same. The flakes obtained from the drum were at a temperature of from about degrees centigrade and had substantially the same size and flowability characteristics as those produced in Example 1.

EXAMPLE 5 The procedure of the preceding examples was repeated with the exception that the drum was cooled to a temperature of about 20 degrees centigrade and was rotated at a speed of 2 rpm, to provide a retention time of about 24 seconds. The flakes obtained were free-flowing and showed only slight evidence of caking during storage.

To illustrate the effects of operating the present process at conditions of time, temperature and layer thicknesses outside those which have been specified hereinabove,

It is of interest to note that the flakes obtained in Example 9 were at a temperature of about 28 degrees centigrade while those obtained in Example 4, which were still free-flowing after five weeks of storage, were obtained from the flaker at a temperature of about 30 degrees centigrade.

What is claimed is:

1. A process for preparing a flaked phosphorous acid product comprising depositing a layer of molten phosphorous acid having a thickness of from about 0.01 to about 0.1 inch on a cooled surface, said surface not being in excess of about 30 degrees centigrade, maintaining said layer on the surface for a period of time of at least about 20 seconds to obtain substantially uniform temperature throughout the layer thickness whereby substantially complete solidification of the phosphorous acid in a stable crystalline form is eflected and thereafter removing the layer from the surface and forming it into flakes, thereby obtaining a substantially non-caking flaked phosphorous acid product.

2. The method as claimed in claim 1, wherein the cooled surface is at a temperature from about 0 degrees centigrade to about 25 degrees centigrade.

3. The method as claimed in claim 2, wherein the layer of a molten phosphorous acid is maintained on the cooled surface for a period of time from about 20 seconds to about 100 seconds.

4. The method as claimed in claim 1, wherein the molten phosphorous acid deposited on the cooled surface is at a temperature within the range of about to degrees centigrade.

5. The method as claimed in claim 1, wherein the cooled surface is moving.

6. The method as claimed in claim 5, wherein the cooled, moving surface upon which the molten phosphorous acid is deposited is a rotating drum, which drum is rotating at a speed not substantially in excess of about 2 revolutions per minute.

References Cited UNITED STATES PATENTS 3,011,878 12/1961 Lackey et al. 23-293 OTHER REFERENCES Voigt et al.; Inorganic Synthesis, vol. IV, 1953), pp. 55-7 relied on.

Perry (Editor); Chem. Engrs. Handbook, third edit. (1950), pp. 1164-66 relied on.

HERBERT T. CARTER, Primary Examiner U.S. Cl. X.R. 23165 

